Competitive displacement of apple aphid by spirea aphid (Homoptera: Aphididae) on apple as mediated by human activities

Survey data from 5 commercially managed apple orchards in West Virginia showed that spirea aphid, Aphis spiraecola Patch, was the more abundant species and that apple aphid, A, pomi De Geer, was present primarily during the late summer when apple is least suitable as a host for aphids. There was no significant positive or negative interaction between the distribution of the 2 species within orchards. In the laboratory, adult apple aphid survived longer at 15 degrees C than spirea aphid but produced the same total number of progeny. At 25 and 28 degrees C, spirea aphids produced more total progeny and more progeny per day than apple aphids. In an unsprayed orchard that had apple aphids introduced in the autumn of 1990, both species were equally abundant in the spring of 1991, but by the end of that summer spirea aphid was the only aphid species present. In a 2-species population model, reproductive differences between the species were sufficient for spirea aphid to dominate the guild quickly. Several different insecticide schedules marginally increased the rate at which spirea aphid replaced apple aphid. The model that most closely resembled field data had a delay in apple aphid immigration and a reduced rate of emigration as compared with spirea aphid. The reduced tendency to emigrate from apple is the one competitive advantage of apple aphid permitting them to remain in the system despite their reproductive disadvantage and greater susceptibility to insecticides. The use of insecticides that produce higher mortality of apple aphid have added to displacement by spirea aphid.